This invention pertains to improved gas generating compositions and more specifically to compositions which produce nitrogen as the predominant gaseous product. While this invention has broad utility, a major use would be for inflating emergency safety devices such as automobile passive restraint systems--namely air bags, aircraft safety chutes and other passenger safety devices.
Because of its inertness, nitrogen is well suited as a gas for inflating emergency safety devices. The nitrogen gas employed in such devices is usually supplied by the chemical reaction of a nitrogen gas generating composition. Since emergency inflating devices must often be extremely compact, low cost and lightweight, a pyrotechnic chemical composition is often used to generate the nitrogen gas. A convenient chemical source of pure nitrogen is sodium azide. Although this well-known substance can be made to release nitrogen if heated to about 750.degree. F. (400.degree. C.), the process is greatly facilitated by the incorporation of oxidants, such as certain metal oxides or inorganic salts.
Other alkali metal azides, such as potassium azide and lithium azide, can also be made to release nitrogen; however, they are more expensive than sodium azide and possess properties which make them less desirable than sodium azide as nitrogen gas generants. At the present time, sodium azide is the preferred source of nitrogen for emergency safety devices such as the widely publicized and debated air bags for automobiles. However, sodium azide presents some extremely serious problems in widespread commercial applications. Sodium azide is extremely toxic if inhaled or ingested by humans or other animals, a property shared with all water-soluble azides. Further, an aqueous solution of sodium azide will react with metal ions of copper, lead and many other metals to produce highly sensitive explosives. Lead azide, for example, is a well-known and sensitive primary explosive while copper azide is deemed far too sensitive to shock, friction, and heat to be of practical use. Sodium azide also reacts with common acids, such as battery acids, to produce hydrazoic acid, a gas with lethality comparable to that of hydrogen cyanide used in gas chambers for execution. Thus, while sodium azide is an effective source of nitrogen for emergency safety devices, it is an extremely dangerous one. Indeed, even after sodium azide has been oxidized in generating nitrogen, danger remains, since the resulting sodium oxide is a well-known caustic.
Since the azides used in gas generating systems such as those for automobile air bags might come in contact with water or battery acid after an accident, during servicing, in junkyards after useful life, or otherwise, it is important that the essential dangers of exposed azides and by-products be minimized insofar as possible.